System and method for recirculating power

Abstract

Methods and systems for operating a driveline that includes two electric machines are described. In one example, one electric machine is operated in a generator mode and the other electric machine is operated in a motor mode so that electric power that is generated by the electric machine that is operated in the generator mode is consumed by the electric machine that is operated in the motor mode.

The invention claimed is: 
     
         1 . A method for operating a vehicle, comprising:
 operating a first electric machine in a motor mode; and   operating a second electric machine in a generator mode, while simultaneously operating the first electric machine in the motor mode, and while a net power amount to and from an electric energy storage device is substantially zero;   where the first electric machine and the second electric machine are each configured to be operable in each of a motor mode and a generator mode; and   where the electric energy storage device is a battery or a capacitor.   
     
     
         2 . The method of  claim 1 , where the first electric machine is coupled to a first axle and the second electric machine is coupled to a second axle. 
     
     
         3 . The method of  claim 1 , where the first electric machine and the second electric machine are coupled to a vehicle's front wheels, and the first electric machine is coupled to the vehicle's front right wheel and controlled independently of the second electric machine coupled to the vehicle's front left wheel, or where the first electric machine and the second electric machine are coupled to the vehicle's rear wheels, and the first electric machine is coupled to the vehicle's rear right wheel and controlled independently of the second electric machine coupled to the vehicle's rear left wheel. 
     
     
         4 . The method of  claim 1 , further comprising reducing a speed of the vehicle via the second electric machine while operating the first electric machine in the motor mode. 
     
     
         5 . The method of  claim 1 , further comprising operating the first electric machine to consume electric power generated via the second electric machine. 
     
     
         6 . The method of  claim 1 , further comprising operating the second electric machine in the generator mode and the first electric machine in the motor mode in response to a state of charge of the electric energy storage device. 
     
     
         7 . A vehicle system, comprising:
 a first electric machine;   a second electric machine;   an electric energy storage device; and   a controller including executable instructions stored in non-transitory memory that cause the controller to operate the first electric machine in a motor mode and the second electric machine in a generator mode with the first electric machine consuming an entire amount of electric energy produced via the second electric machine.   
     
     
         8 . The vehicle system of  claim 7 , further comprising a first gearbox coupled to the first electric machine, a second gearbox coupled to the second electric machine, and a tandem axle, where the first electric machine selectively provides propulsive effort to the tandem axle. 
     
     
         9 . The vehicle system of  claim 8 , further comprising additional instructions to shift the first gearbox and the second gear box to provide a desired level of vehicle braking. 
     
     
         10 . The vehicle system of  claim 8 , further comprising additional instructions to shift the second gearbox to a numerically higher gear ratio than the first gearbox to increase vehicle braking torque. 
     
     
         11 . The vehicle system of  claim 7 , where the first electric machine is coupled to a first axle including a first final drive gear set, and where the second electric machine is coupled to a second axle including a second final drive gear set. 
     
     
         12 . The vehicle system of  claim 11 , where the first final drive gear set has a different gear ratio than the second final drive gear set. 
     
     
         13 . The vehicle system of  claim 11 , where the first axle is a rear axle and the second axle is a front axle. 
     
     
         14 . The vehicle system of  claim 7 , where the first electric machine is operated in the motor mode and simultaneously the second electric machine is operated in the generator mode in response to a temperature of the electric energy storage device. 
     
     
         15 . A method for operating a vehicle, comprising:
 reducing a speed of the vehicle via a second electric machine, coupled to a second axle, in response to a braking request without providing electric power from the second electric machine to an electric energy storage device;   where a first electric machine, coupled to a first axle, consumes the electric power generated by the second electric machine while reducing the speed of the vehicle via the second electric machine.   
     
     
         16 . The method of  claim 15 , where the second electric machine is operated in a generator mode to provide a negative torque to a driveline while the first electric machine is operated in a motor mode to provide a positive torque to the driveline, and where the negative torque is larger than the positive torque. 
     
     
         17 . The method of  claim 15 , further comprising adjusting a gear ratio of a gearbox to adjust an efficiency of the second electric machine, the gearbox coupled to the second electric machine. 
     
     
         18 . The method of  claim 15 , further comprising reducing the speed of the vehicle via the second electric machine in response to a temperature of the electric energy storage device. 
     
     
         19 . The vehicle system of  claim 7 , further comprising additional instructions to operate the first electric machine in the motor mode and the second electric machine in the generator mode in response to a state of charge of the electric energy storage device being greater than a threshold. 
     
     
         20 . The method of  claim 15 , further comprising reducing the speed of the vehicle via the second electric machine and the first electric machine in response to the braking request when a state of charge of the electric energy storage device is less than a threshold, where the speed of the vehicle is reduced via the second electric machine in response to the braking request without providing electric power from the second electric machine to the electric energy storage device when the state of charge of the electric energy storage device is greater than the threshold.